The present invention relates to equipment for leading a web threading tail in a paper machine, which equipment includes at least two sequential surfaces in the direction of travel of the web threading tail, between which a nozzle is arranged in order to form a directed air blast and thus to transport the web threading tail, being lead onto the first surface in the direction of travel of the web threading tail, onwards to the following surface.
In paper machines and in other web-forming machines, there are consecutive processing stages, through which the web is transferred with the aid of a web threading tail. In practice, the narrow web threading tail is first of all taken through the paper machine, after which it is spread out to form a full-width web. Web threading can also consist of several different stages. In a processing stage, the web threading tail is transported, for example, with the aid of web-threading ropes. However, between the processing stages there are breaks in the web threading devices, when various apparatuses are used to transport the web threading tail.
Various blast plates are known for leading the web threading tail. The operation of a blast plate is based on the Coanda phenomenon, which is created by arranging a blast of air parallel to the surface of the blast plate. A vacuum arises on the surface and sucks the web threading tail in the direction of the surface. At the same time, the blast of air pushes the web threading tail forward. If necessary, the blast plate can even be made extremely small, without any moving parts. On the other hand, connecting several blast plates in sequence will create an apparatus forming a transport path for the web threading tail.
Existing equipment based on blast plates has many problems and deficiencies. In known equipment, the air blast is created using high-pressure nozzles. In other words, the blast plates operate at an air pressure that is normally in the range of 2-6 bar (200-600 kPa), depending on the paper mill's compressed-air network. In addition, about 0.05 m3/s of compressed air is needed for each nozzle. As there can be 10-30 nozzles in a single apparatus, the consumption of compressed air is significantly large. In practice, a nozzle is formed from a duct, in which holes are machined close to each other. The holes are at intervals of about 10-20 mm and have a diameter of about 1.5 mm. The air discharges from the holes at the speed of sound. In practice, despite increasing the pressure, the speed cannot be increased above this. Though an increase in pressure will slightly increase the rate of movement of the air, in practice the increase will nevertheless be insignificant.
Air discharging from a hole at the speed of sound causes a great deal of noise. In addition, the pressure in the nozzle cannot be adjusted, should the speed of the web threading tail or the grade of paper change. In practice, the transporting force is always the same, which makes it difficult to apply the equipment to different positions. Further, the high-speed air blast creates a high vacuum precisely at the nozzle and especially after it. In practice, the web threading tail then tends to be sucked onto the surface preceding the nozzle. This abrasion creates much friction, which hampers the onward transportation of the web threading tail.
Particularly in long apparatuses, the distance between the nozzles in the machine direction is often too great. In practice, the velocity of the air blast drops rapidly after the vacuum peak following the nozzle. Thus, the force in the equipment transporting the web threading tail varies wildly, hampering web threading. In other words, the traction is discontinuous. In addition, the high-velocity air blast can even break the web threading tail. Further, known equipment lacks a force correcting the web threading tail, should the web threading tail deviate laterally from the planned path.